1) Another way to say "hurry up".
2) An expression of disbelief, similar to "what's up with that?".
3) To ejaculate on an object or person.
George: "Come on, we're gonna be late!"
Shirley: "Hold on, I've gotta find a new dress to wear. My old one got come on."
George: "Come on! You're such a dumb slut!"
After it became known that the phrase was used to rectify the long-standing altercation between the Bloods and the Crips, people told the Israelis and Palestinians to just "come on", and they all finally came to their senses. A few months later, the guys working for a New World Order were told to come on, and they instantly lost interest in their plans for world domination. Later everyone got together and smoked a bowl.